Variable resistor



Nov. 1, 1960 W. W. GARSTANG VARIABLE RESISTOR 2 Sheets-Sheet 1 FiledApril 15, 19 59 ATTORNEY N V- 1, 9 w. w. GARSTANG 2,958,841

VARIABLE RESISTOR Filed April 15, 1959 2 Sheets-Sheet 2 INVENTORWAMMLMK; awn/{M ATTORNEY Unite atent 2,958,841 Patented Nov. 1, 1960VARIABLE nnsrsron William W. Garstang, Fox Point, Wis., assignor toAllen- Bradley Company, Milwaukee, Wis., a corporation of WisconsinFiled Apr. 13, 1959, Ser. No. 805,945

1 Claim. Cl. 338-180) This invention relates to linear variableresistors and it more specifically resides in a resistor having aselfsustaining body that is of molded composition material withconducting particles dispersed therein whereby the body itselfconstitutes the electrical resistance path of the resistor, and incombination with the body there is provided satisfactory terminals and asliding contact which engages and moves along the length of the body.

In the assembly of electronic circuits substantial progress has beenmade in devising means of simplifying and standardizing circuitconstruction to achieve reduced costs and labor. Variable components inthese circuits constitute some of the more intricate elements thereof,and therefore account for a considerable portion of the costs. This canbe particularly objectionable where the circuits are intended for massproduction and distribution, or for short life where expendability isdependent upon low initial costs. The present invention, whileapplicable to highly precise variable resistor components of preciseworkmanship, also lends itself to the provision of a variable resistorthat may be economically manufactured to provide low unit cost. Also, aresistor is provided that is readily inserted in a chassis or printedcircuit board, so that its assembly into a circuit network may beefficiently and rapidly carried out.

A feature of the invention comprises a self-sustaining body which itselfis the resistance path of the resistor. The body is formed of a hotmolded resin, which is an insulating material, and conductive particlesare dispersed throughout the body proper to provide a conductive path ofhigh resistance. The body is preferably formed in a tubularconfiguration, so that the interior may house an operating shaft with amovable contact thereon for adjustment of the resistance value. To mountthe resistor, and provide a bearing for the shaft, a simple cap may befastened on one end of the body. The cap not only presents mountingmeans, but also locates the shaft, thereby achieving a variable resistorwith a minimum of parts. compactness and simplicity of design resultsfrom the novel construction of a body that is not merely a housing for aseparable resistance element, but in which the body itself constitutesthe resistance element.

It is an object of this invention to provide a variable resistor ofrelatively few parts which may be economically manufactured.

It is another object of this invention to provide a variable resistorthat is rugged so as to withstand mechanical abuse.

it is another object of this invention to provide a variable resistorthat has excellent electrical characteristics such as stability ofresistance value under varying load conditions and for a long lifeperiod.

It is another object of this invention to provide a variable resistorthat is adapted to be readily inserted into a printed circuit network.

The foregoing and other objects and advantages of this invention willappear from the following description. In the description reference ismade to the accompanying drawings, which form a part hereof, and inwhich there is shown by way of illustration and not limitation specificform in which the invention may be embodied.

In the drawings:

Fig. l is a view in longitudinal cross section of a linear variableresistor embodying the invention which may be used either as apotentiometer or a rheostat,

Fig. 2 is a fragmentary View in longitudinal cross section of theresistor shown in Fig. 1 taken on the plane 2-4,

Fig. 3 is a fragmentary side view in elevation of the resistor shown inFig. 1,

Fig. 4 is a top view of the resistor of Fig. 1,

Fig. 5 is a bottom view of the resistor of Fig. 1,

Fig. 6 is a view in longitudinal cross section of a second embodiment ofthe invention in which the resistor is mounted upon a thin board,

Fig. 7 is a top view of the resistor and boards shown in Fig. 6,

Fig. 8 is a view in transverse cross section of the resistor in Fig. 6viewed through the plane 8-8,

Fig. 9 is a fragmentary view in longitudinal cross section of the upperportion of another form of resistor embodying the invention, and

Fig. 10 is a top view of the resistor shown in Fig. 9.

Referring now to the drawings, there is shown in Fig. 1 a linearvariable resistor 1 having a circular cylindrical body 2 with an axiallyextending central bore 3 passing through the entire length of the body2. A pair of diametrically opposed grooves 4 are recessed in the sidesof the bore 3 and likewise extend for the entire axial length of thebody 2. The body 2 is of a sufiicient wall thickness to lend mechanicalrigidity to the body, whereby it is a self-sustaining structural memberof rugged characteristics to withstand abusive handling and mechanicalshock to which electrical circuit components are frequently subject. Thebody 2 structurally comprises the main element of the resistor 1, and itfunctions to support the remaining structure of the resistor, as will bedescribed herein.

It is a purpose of this invention to provide a body portion for aresistor that not only is the principal structural unit for theresistor, but which is also a conductive path forming the resistanceelement of the resistor. To this end, the body 2 is preferably formed ofa hot molded composition resin such as a phenol-aldehyde condensationproduct together with appropriate filler, lubricant, and otheringredients if desired. A typical material of this nature is describedin US. Letters Patent No. 2,269,- 136 dated January 6, 1942 to B. F.Tellkarnp. IntermiXed with the resinous material of the body 2, so as tobe dispersed throughout the same, are small electrically conductingparticles and a material ideally suitable for this purpose is finelycomminuted carbon black. The technique of intermixing the particles withthe resin is discussed in said Letters Patent. The resulting body 2,then, is a single integral structure that is a self-sustainingresistance element without separate or separable resistance unitsindividually attached thereto.

A pair of terminal leads 5 and 6 are inserted in the body 2 at the timeof molding, and extend axially there from at opposite ends of the body 2to provide for electrical connection with the resistance elementpresented by the body 2. A cylindrical ring 7 of insulating materialtightly fits about the upper end of the outer surface of the body 2. Thering 7 may be adhesively secured in place, or if desired may be anintegrally molded extension of the body 2. Covering the upper end of thebody 2 is an insulating washer 8, which is held in place by a metalliccap 9 that is in clipped engagement with the ring 7. The manner ofclipping the cap 9 in position is shown in Figs. 2 and 3, wherein it isseen that a pair of ears 10 and 11 project downwardly on opposite sidesof the body 2 which have, respectively, upwardly extending springfingers 12 and 13 that are bent inwardly so as to engage the lower edgeof the insulating ring 7. The fit of the cap 9 is such as to tightlyhold the washer 8 in position. The cap 9 has a slot 14 at the pointwhere the terminal lead emerges from the body 2, so as to beelectrically separate therefrom, and on the side of the cap 9 oppositethe slot 14 is a formed terminal 15 suitable for mounting the resistorl. and for making electrical engagement with a sliding contact to bedescribed. 7

A threaded adjustment screw 16 extends downwardly through the interiorof the body 2 and has a collar 17 at its upper end that is held betweenthe washer 8 and the top of the cap 9. In this manner the screw 16 isheld captive to preclude axial movement, but it is free to rotate. Ahead 18 extends from the top of the screw 16, and projects above the cap9 to provide means for rotating the screw 16. As is more clearly shownin Figs. 3 and 4, the upper surface of the cap 9 comprises a set ofcircumferentially arranged radially inwardly extending spring fingers19. The fingers 19 bear downwardly upon the collar 17 of the screw 16,and the resilience of the fingers 19 insures a positive retention of theadjustment screw 16.

Mounted upon the adjustment screw 16 is a sliding contact 20 which has apair of contact points 21, 22 that ride in the grooves 4 and bearagainst the inner wall of the body 2, so as to be in electricalengagement therewith. The contact 2t? is held from rotation by reason ofthe grooves 4, and is engaged with the threads of the adjustment screw16, whereby upon rotation of the screw 16 the contact 20 will be causedto slide axially along the resistance path presented bythe body 2.

The resistor 1 may be placed in circuit either as a potentiometer orrheostat. For potentiometer connection the terminals 5 and 6 are joinedto a source voltage, and the terminal 15 together with one of the otherterminals 5, 6 serve to present an output. For functioning as arheostat, the terminal 15 and one of the other terminals 5, 6 areconnected in the circuit. The adjusting screw 16 and cap 9 consequentlyfunction as conducting elements of the resistor 1, as well as serving asstructural portions. The resistor 1 has relatively few parts in itsassembly, with all major constituents acting as current conductingmembers as well as serving some structural purpose. The resistoraccordingly provides a construction of economical manufacture while atthe same time providing a hard, rugged body that may withstand severemechanical abuse. Phenolic products are commonly used in electricalapparatus as structural members because of recognized physical strength,and in this invention such mechanical strength is availed of while atthe same time utilizing the principal body as the resistance element.

Turning now to Figs. 68, there is shown another embodiment of theinvention, which is particularly adaptable for application as arheostat. A tubular body 23 comprising a resistive element and formedsimilarly as the body 2 of Figs. 1-5 has a closed base 24 in which isembedded one end of a terminal lead 25. The base 24 is an integralcontinuation and is a part of the body 23 and therefor is alsoconductive. The terminal lead 25 is consequently in electricalconnection with one end of the resistance path presented by the body 23.Fitted over the top of the body 23 is a cap 26 having a plurality ofdownwardly extending spring arms 27 that engage a groove 28 extendingcircumferentially about the outer surface of the body 23. The springarms 27 thus grasp the upper end of the body 23 to hold the cap 26 inplace, and at the center of the cap 26 is an axially extending neck 29that is internally threaded to receive and hold an adjustment screw 30.By turning the screw 30 it is worked upwardly or downwardly in the neck29. Riveted to the lower end of the screw St is a pick up contact 31formed with a plurality of spring fingers 32, as more clearly shown inFig. 8, which slide against and are in electrical engagement with theresistance track afforded by the body 23. In the form of the inventionof Fig. 6, guiding grooves in the inner wall of the body are eliminatedand the contact 31 is free to rotate with the adjustment screw 30. Theconnection of the screw 30 and contact 31 tightly presses these twoelements together so that current may readily be conducted from the body23 through the contact 31 to the adjustment screw 30.

As shown in Figs. 6 and 7 the resistor is mounted in an opening 33 of aprinted circuit board 34. The neck 29 is extended through the opening 33and the flat upper side of the cap 26 bears against the underside of theboard 34. A spring clip 35 formed of sheet material fits over the neck29 and engages the upper side of the board 34. The spring clip 35 has apair of arms 36 that tightly engage the neck 29 to be in firm frictionalconnection therewith, and also to be in electrical contact therewith.The lower outer corners of the spring clip 35 are formed with pointedengagement toes 37 that press into the upper side of the board 34. Theupper side of the board 34 has a conductive overlay 38 that is a part ofthe wiring of a printed circuit, and the toes 37 engage the overlay 38,whereby the resistor becomes a part of the circuit. Hence, the springclip 35 serves to hold the resistor in place and also to complete anelectrical connection.

In Figs. 9 and 10 an alternative form of cap 39 is shown which hasdownwardly extending spring arms 4 for engaging a resistor body 41 and asecond set of radially extending spring arms 42 which are used inmounting the resistor. In mounting the resistor of Figs. 9 and 10, theresistor is inserted downwardly through an opening in the mounting board43, and the board 43 is held between the spring arms 42 and a set ofknuckles 44 on the spring arms 40. The cap 39 retains an adjustmentscrew 45 in similar fashion as the cap 9 of Figs. 15.

The snap type cap constructions shown in the draw ings are held inassembly through spring pressures. They provide a means of easilymounting a complete resistor in a circuit by simple snap-in connectionand such caps in combination with the conductive resistor bodies of theinvention provide advantageous resistor constructions wherein a variableresistor has a minimal number of parts. The particular embodiments ofthe drawings are not intended as being definitive of the scope of theinvention, but rather as a showing of representative forms which theinvention may take. Reference is had to the following claim which isintended to more fully state and definie the scope, said claim beingdeemed inter alia as entitled to a range of equivalents.

I claim:

In a variable linear resistor the combination of a hollow cylindricalbody open at one end and formed of molded material with conductiveparticles dispersed therein to provide a resistance path; a terminaljoined to the body and extending therefrom; a bearing plate over theopen end of the hollow body that has a central opening and forms abearing support for an operating screw; an operating screw extendingthrough the central opening of said bearing plate into the interior ofsaid hollow body, with an enlarged collar resting upon said bearingplate and a head extending outwardly from the collar: a cap of thinspring material covering the bearing plate and operating screw collarhaving resilient fingers in clip engagement with the hollow body andhaving resilient portions bearing downwardly upon said operating screwcollar that retain said operating screw in position; and an axiallymovable contact within said hollow body mounted upon said operatingscrew.

Rubinstein Oct. 31, 1939 Payne Sept. 13, 1949

